ABB monitors silica at power plant to cut NOx emissions

March 17, 2010 - A new high pressure water injection system designed to reduce nitrogen oxide emissions from a peaking power plant in Holtsville, NY requires water with an ultra-low silica concentration. The twin-pack Pratt & Whitney kerosene-fired engines have a limit of 20 ppb silica in fuel and water entering the turbines. Higher silica values can coat the turbines, leading to decreased efficiency, engine damage, and costly downtime. Estimates for replacing damaged parts for the plants engines range as high as $5 million.

National Grid chose the new Navigator 600 silica analyzer from ABB to monitor silica in the water to be injected. The Navigator 600 analyzer needs little operator attention plus reagent usage is comparably low. The analyzer is self-calibrating and self-cleaning, and provides a historical data log of alarms and events.

National Grid installed the new $4 million high-pressure water injection system and silica analyzer at its largest peaking generation facility on Long Island. The water system automatically injects and mixes ultra-pure, high-pressure water into the kerosene fuel before it reaches the combustion chambers of the aircraft engines. The injected water lowers the peak flame temperature in the engine, decreasing NOx emissions by 50 to 60%.

A trailer with ion exchange resin beds purifies incoming city water. The water flows through a four-inch line to a 500,000-gallon storage tank. Originally, the station relied on a conductivity monitor to gauge when the resin beds in the trailer were exhausted. This monitor was contained within the trailer and would automatically shut off the water when the conductivity rose to 0.5 micro-mhos.

The ABB silica analyzer now checks on the silica levels of water entering the storage tank. These levels can rise dramatically as the resin beds in the trailer near their limit and are exhausted. In fact the Navigator 600 analyzer initially indicated readings of about 1600 ppb during a trial period.

The large amount of water in the tank dilutes the silica concentration, but the analyzer showed tank ppb silica levels in the teens during a backflush test. These values met the engine manufacturer's specification but were uncomfortably near the limit.

With the ABB silica analyzer now monitoring the water entering the storage tank, Holtsville will be shutting off the DI trailer and ordering another when the silica level in the line to the storage tank rises to 50 ppb during a fill cycle. An alarm contact on the Navigator 600 analyzer will automatically initiate shut off of water flow and energize a beacon strobe light to alert the operators.

To further diminish NOx emissions, National Grid plans to expand the concept of the HP water injection system to similar peaking stations within their system around Long Island.

ABB is a leader in power and automation technologies that enable utility and industry customers to improve their performance while lowering environmental impact. The ABB Group of companies operates in around 100 countries and employs about 107,000 people.